Intrinsic uncertainty in the sub-daily satellite products at their native resolutions

For many hydrological and weather forecasting applications, an important quantity is the amount of precipitation that falls on the Earth’s surface over a given time interval, i.e., the surface precipitation rate. However, no satellite instrument is unambiguously sensitive to the instantaneous precipitation rate at the Earth’s surface. A vertically profiling radar such as the Dual-Frequency Precipitation Radar (DPR) onboard the joint National Aeronautics and Space Administration (NASA) and Japanese Aerospace Exploration Agency (JAXA) Global Precipitation Measurement (GPM) mission satellite (Hou et al., 2014) is directly sensitive to rain in the near-surface layers of the atmosphere unaffected by surface ground clutter, but its measurements are affected by attenuation due to the condensed water in the higher layers of the cloud. Furthermore, the DPR instrument scan swath is typically too narrow (240 km) to provide substantial global coverage at sub-weekly scales. Passive microwave (MW) radiometers do have more substantial coverage owing to their wider swath (between 800 and 2500 km). These measurements are less directly sensitive to surface rain, with more direct sensitivity to the condensed water in the cloud. The height of the “peak” sensitivity increases as the radiometer channel wavelength decreases, up to infrared (IR) radiometers, which are directly sensitive only to the condensed water at the very top of the cloud (Haddad et al., 2017). IR radiometers are now carried by all meteorological geostationary satellites, providing global coverage with very frequent temporal sampling (at least every 30 minutes or better).

对于许多水文和天气预测应用而言，一个重要的量是在给定时间间隔内落在地球表面的降水量，即地表降水率。然而，没有任何卫星仪器对地球表面的瞬时降水率有明确的敏感性。像美国国家航空航天局（NASA）和日本宇宙航空研究开发机构（JAXA）联合的全球降水测量（GPM）任务卫星上的双频降水雷达（DPR）这样的垂直剖面雷达，对不受地表杂波影响的大气近地表层的降雨直接敏感，但其测量会受到云层较高层的凝结水造成的衰减影响。此外，DPR仪器的扫描幅宽通常太窄（240千米），无法在亚周尺度上提供大量的全球覆盖。被动微波（MW）辐射计由于其更宽的幅宽（在800到2500千米之间）确实有更广泛的覆盖范围。这些测量对地表降雨的直接敏感性较低，对云层中的凝结水更直接敏感。随着辐射计通道波长的减小，“峰值”敏感性的高度会增加，直至红外（IR）辐射计，它仅对云层顶部的凝结水直接敏感（Haddad等人，2017年）。现在所有的气象静止卫星都搭载红外辐射计，以非常频繁的时间采样（至少每30分钟或更短时间）提供全球覆盖。